Many fiber optic systems have operational characteristics that are highly dependent on the polarization of the light guided by the optical fiber. Such systems include optical angular rate sensors, herein referred to as fiber optic gyros, and interferometric sensors. Such sensors generally require that a single polarization state be maintained in order to achieve desired accuracy and performance characteristics.
Use of polarization-maintaining fibers to maintain single polarization state operation in fiber optic gyros have not proven to be adequate to achieve the purity of single polarization required for high performance goals. This is so since polarization noise directly impacts the bias and bias instability in fiber optic gyros. The bias and bias instability are generally attributed to polarizaiton coupling in the fiber loop in fiber optic gyros. Accordingly, there exists a need for a fiber optic polarizer having high extinction ratio and low insertion loss characteristics.
Several types of fiber optic polarizers which are based upon the interaction between the evanescent field of the guided wave and the material in the evanescent field region have been demonstrated. Polarizers of high extinction ratio and low loss are currently implemented using either birefringent crystal or metal films. In a publication entitled, "Single-Mode Fiber Optic Polarizer," Opt. Lett., Vol. 5, No. 11, pp. 479-481, 1980, by R. A. Bergh, H. C. Lefevre, and H.J. Shaw, a crystal polarizer was shown incorporating a birefringent crystal. The crystal was placed on a length of fiber from which a portion of cladding had been removed. The evanescent field of the light guided by the fiber interacts with the birefringent crystal causing the light of unwanted polarization to couple out of the fiber and into the birefringent medium where it is no longer guided by the fiber. However, the light of desired polarization is unaffected by the birefringent crystal and remains guided by the fiber.
Metal-clad fiber optic polarizers based on either the differential attenuation of the two polarization modes or the cut-off of the TE.sub.O mode have been described and demonstrated in previous work as described in publications entitled "Fabricaton of Single-Mode Fiber-Type Polarizer," Opt. Lett., Vol. 8, No. 2, pp. 124-126, 1983 by T. Hosaka, K. Okamoto and T. Edahiro, and "Metal-Clad Fiber-Optic Polarizer," Opt. Lett., Vol. 11, No. 6, pp. 386-388, 1986 by J.R. Feth and C.L. Chang, respectively. All of the above mentioned references being incorporated herein by reference.
In addition to low loss and high extinction ratio, an environmentally stable and wavelength-independent polarizer is desired for practical applications. The crystal polarizer is inherently very sensitive to the temperature variation because the refractive index changes with temperature. Furthermore, the birefringent crystal should be either mechanically or adhesively bonded to the optical fiber where the cladding material has been removed. The resultant device thus suffers very severe mechanical and thermal instabilities. The metal-clad fiber polarizer based upon the differential attenuation of the two polarizations requires a very thick metal film which is prepared very close to the fiber core. High extinction ratio and low loss cannot be achieved simultaneously with this type of device. The other metal clad polarizer that utilizes a very thin metal film could be sensitive to the operating wavelength, although high extinction ratio and low insertion loss can be obtained.